Power supply circuit with pfc function, and automatic gain control circuit therefor and control method thereof

ABSTRACT

The present invention discloses a power supply circuit with power factor correction (PFC) function, and an automatic gain control circuit therefor and a control method thereof. The power supply circuit includes the automatic gain control circuit and a load driver circuit. The automatic gain control circuit converts an input voltage to a regulation voltage, and the load driver circuit generates an output current according to the regulation voltage. The automatic gain control circuit automatically adjust the regulation voltage such that the regulation voltage has a substantially fixed amplitude or fixed average value under different input voltages of different specifications, and the output current provided by the load driver circuit varies in phase with the input voltage to provide a PFC function.

CROSS REFERENCE

The present invention claims priority to U.S. provisional applicationNo. 61/615,263, filed on Mar. 24, 2012.

FIELD OF INVENTION

The present invention relates to a power supply circuit with powerfactor correction (PFC) function, and an automatic gain control circuittherefor and a control method thereof; particularly, it relates to suchpower supply circuit, and automatic gain control circuit therefor andcontrol method with automatic gain control and PFC function.

DESCRIPTION OF RELATED ART

FIG. 1 shows a schematic diagram of a prior art power supply circuit100. As shown in FIG. 1, a voltage Vac is for example an AC voltagesignal, which is to be rectified; however, the voltage Vac can be a DCvoltage signal. A rectifier circuit 101 rectifies the voltage Vac togenerate an input voltage Vin1. The rectifier circuit 101 for example isa bridge rectifier circuit. The power supply circuit 100 includes a PFCcircuit 102, a power stage circuit 103, and a control circuit 105. ThePFC circuit 102 converts the input voltage Vin1 to an input voltage Vin2and outputs a current Iin2, wherein the current Iin2 is in phase withthe input voltage Vin2 (i.e., the timings when the amplitude of thecurrent Iin2 reaches its peak and valley are the same as the timingswhen the amplitude of the input voltage Vin2 reaches its peak andvalley), to improve the power factor of the power conversion. The PFCcircuit 102 is typically a power converter circuit which includes one ormore power switches and an inductor, which is well known by thoseskilled in the art, so the details thereof are omitted here. The controlcircuit 105 generates a driving signal GATE for operating a power switchin the power stage circuit 103 to convert the input voltage Vin2 to anoutput voltage Vout and to provide an output current Iout to a load (notshown). The prior art power supply circuit 100 has a drawback of arelatively high manufacturing cost because it requires the PFC circuit102, and the PFC circuit 102 includes expensive components such as thepower switch and the inductor. If the PFC circuit 102 is omitted, themanufacturing cost can be reduced significantly.

Besides the PFC function, because the specification of the voltage Vacis different (such as 110V or 220V) in different countries, it ispreferable for the power supply circuit to be capable of generating thesame output current Tout (i.e., with the same amplitude or averagevalue) even if the voltages Vac (or the rectified input voltages Vin1)is different.

In view of above, to overcome the drawbacks in the prior art shown inFIG. 1, the present invention proposes a power supply circuit, and anautomatic gain control circuit therefor and a control method thereof,which can generate an output current with the same amplitude or the sameaverage value under different input voltages, the output current and theinput voltage Vin are in phase.

SUMMARY OF THE INVENTION

From one perspective, the present invention provides a power supplycircuit, for generating an output current according to an input voltage,wherein the output current has a substantially same average value underdifferent input voltages of different specifications, the power supplycircuit including: an automatic gain control (AGC) circuit, forgenerating a regulation signal which is in phase with the input voltage,wherein the regulation signal has a substantially same amplitude oraverage value under different input voltages of differentspecifications; and a load driver circuit, controlled by the regulationsignal with an open loop connection, for generating the output current.

In one preferable embodiment, the load driver circuit includes a controlcircuit and a power stage circuit, wherein the control circuit receivesthe regulation signal with an open loop connection and generates adriving signal for controlling the power stage circuit to generate theoutput current according to the regulation signal.

In one preferable embodiment, the automatic gain control circuitincludes: a voltage-to-current converter circuit for converting an inputvoltage to a regulation current; a current-to-voltage converter circuit,which is coupled to the voltage-to-current converter circuit, forconverting the regulation current to a regulation voltage; a low-passfilter circuit, which is coupled to the current-to-voltage convertercircuit, for obtaining a low frequency signal from the regulationvoltage; and a comparison circuit, which is coupled to the low-passfilter circuit, for generating a control signal according to the lowfrequency signal and a reference voltage to control a parameter in thevoltage-to-current converter circuit, such that the regulation voltagemaintains at a substantially same amplitude or average value underdifferent input voltages of different specifications. And the regulationsignal is the regulation voltage or a signal derived from the regulationvoltage.

In another preferable embodiment, the automatic gain control circuitincludes: a voltage-to-current converter circuit for converting an inputvoltage to a regulation current; a current-to-voltage converter circuit,which is coupled to the voltage-to-current converter circuit, forconverting the regulation current to a regulation voltage; a low-passfilter circuit, which is coupled to the current-to-voltage convertercircuit, for obtaining a low frequency signal from the regulationvoltage; and a comparison circuit, which is coupled to the low-passfilter circuit, for generating a control signal according to the lowfrequency signal and a reference voltage to control a parameter in thecurrent-to-voltage converter circuit, such that the regulation voltagemaintains at a substantially same amplitude or average value underdifferent input voltages of different specifications. And the regulationsignal is the regulation voltage or a signal derived from the regulationvoltage.

In another preferable embodiment, the comparison circuit is adifferential amplifier circuit.

From another perspective, the present invention provides a power supplycircuit, including: an automatic gain control circuit, including: avoltage-to-current converter circuit for converting an input voltage toa regulation current; a current-to-voltage converter circuit, which iscoupled to the voltage-to-current converter circuit, for converting theregulation current to a regulation voltage; a low-pass filter circuit,which is coupled to the current-to-voltage converter circuit, forobtaining a low frequency signal from the regulation voltage; and acomparison circuit, which is coupled to the low-pass filter circuit, forgenerating a control signal according to the low frequency signal and areference voltage; and a load driver circuit, which is coupled to thecurrent-to-voltage converter circuit, for converting the input voltageto the output voltage and supplying an output current, wherein theoutput current is in phase with the regulation voltage; wherein thecontrol signal is inputted to the voltage-to-current converter circuitor the current-to-the voltage converter circuit, for adjusting aconversion ratio from the input voltage to the regulation current, orfrom the regulation current to the regulation voltage.

From another perspective, the present invention provides an automaticgain control circuit of a power supply circuit, wherein the power supplycircuit is for converting an input voltage to an output voltage andsupplying an output current, wherein the output current is in phase withthe input voltage, the automatic gain control circuit including: avoltage-to-current converter circuit, for converting the input voltageto a regulation current; a current-to-voltage converter circuit, whichis coupled to the voltage-to-current converter circuit, for convertingthe regulation current to the regulation voltage; a low-pass filtercircuit, which is coupled to the current-to-voltage converter circuit,for obtaining a low frequency signal from the regulation voltage; and acomparison circuit, which is coupled to the low-pass filter circuit, forgenerating a control signal according to the low frequency signal and areference voltage; wherein the control signal is inputted to thevoltage-to-current converter circuit or the current-to-the voltageconverter circuit, for adjusting a conversion ratio from the inputvoltage to the regulation current, or from the regulation current to theregulation voltage.

In one preferable embodiment, the voltage-to-current converter circuitincludes: a differential amplifier circuit, for generating adifferential amplified signal according to the input voltage and aregulation current feedback signal; a transistor, which is coupled tothe differential amplifier circuit, for operating to generate theregulation current according to the differential amplified signal; and avariable resistor circuit, which is coupled to the transistor, forgenerating the regulation current feedback signal according to theregulation current, wherein a resistance of the variable resistorcircuit is controlled by the control signal.

In the aforementioned embodiment, the variable resistor circuit mayinclude a metal oxide semiconductor (MOS) device, which has a resistancecontrolled by the control signal.

In the aforementioned embodiment, the current-to-voltage circuit mayinclude: a current mirror circuit, for generating a duplicatedregulation current by duplicating the regulation current; and animpedance circuit, which is coupled to the current mirror circuit, forconverting the duplicated regulation current to the regulation voltage.

In another preferable embodiment, the current-to-voltage convertercircuit includes: a current mirror circuit, for generating a duplicatedregulation current by duplicating the regulation current; and a variableresistor circuit, which is coupled to the current mirror circuit, forconverting the duplicated regulation current to the regulation voltage,wherein the variable resistor circuit has a resistance controlled by thecontrol signal.

In the aforementioned embodiment, the variable resistor circuit mayinclude a metal oxide semiconductor (MOS) device, which has a resistancecontrolled by the control signal.

In another preferable embodiment, the voltage-to-current convertercircuit further includes an upper limit gain determination circuit,which is connected to the variable resistor circuit in series, fordetermining an upper limit of a gain of the automatic gain controlcircuit.

In another preferable embodiment, the voltage-to-current convertercircuit further includes a lower limit gain determination circuit, whichis connected to the variable resistor circuit in parallel, fordetermining a lower limit of a gain of the automatic gain controlcircuit.

In another preferable embodiment, the current mirror circuit furtherincludes a constant current source, for determining a minimum level ofthe regulation voltage.

In another preferable embodiment, the load driver circuit includes: adifferential amplifier circuit, for generating a differential amplifiedsignal according to the regulation voltage and a current sense signal; apower stage circuit, for operating at least one power switch thereinaccording to the differential amplified signal, to convert the inputvoltage to the output voltage and to supply an output current; and acurrent sense circuit, which is coupled to the differential amplifiercircuit, for generating the current sense signal according to the outputcurrent or a signal which is related to the output current.

From another perspective, the present invention provides a controlmethod of an automatic gain control circuit for use in a power supplycircuit, wherein the power supply circuit is for converting an inputvoltage to an output voltage, and supplying an output current, whereinthe output current is in phase with the input voltage, the controlmethod including: converting the input voltage to a regulation current;converting the regulation current to a regulation voltage; filtering theregulation voltage to obtain a low frequency signal; and generating acontrol signal according to the low frequency signal; wherein thecontrol signal is for adjusting a conversion ratio from the inputvoltage to the regulation current, or from the regulation current to theregulation voltage.

In one preferable embodiment, the control method further includes:determining an upper limit conversion ratio and/or a lower limitconversion ratio from the input voltage to the regulation current todetermine an upper limit of a gain and/or a lower limit of a gain of theautomatic gain control circuit.

In another preferable embodiment, the control method further includes:determining an upper limit conversion ratio or a lower limit conversionratio from the regulation current to the regulation voltage to determinean upper limit of a gain or a lower limit of a gain of the automaticgain control circuit.

In another preferable embodiment, the control method further includes:determining a minimum level of the regulation voltage.

The objectives, technical details, features, and effects of the presentinvention will be better understood with regard to the detaileddescription of the embodiments below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of a prior art power supply circuit100.

FIG. 2A shows a first embodiment of the present invention.

FIG. 2B shows a second embodiment of the present invention.

FIG. 3 shows a third embodiment of the present invention.

FIG. 4 shows a fourth embodiment of the present invention.

FIG. 5 shows a fifth embodiment of the present invention.

FIG. 6 shows a sixth embodiment of the present invention.

FIG. 7 shows a seventh embodiment of the present invention.

FIG. 8 shows an eighth embodiment of the present invention.

FIGS. 9A-9K show synchronous and asynchronous buck, boost, inverting,buck-boost, inverting-boost, and flyback power stage circuits.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2A shows a first embodiment of the present invention. As shown inFIG. 2A, it is not required for the power supply circuit 200 to includea PFC circuit; the PFC circuit is omitted in a power supply circuit 200.The power supply circuit 200 includes a power stage circuit 203 and acontrol circuit 205, wherein the control circuit 205 generates a drivingsignal GATE, for operating at least one power switch SW1 in the powerstage circuit 203 to convert an input voltage Vin to an output voltageVout. The power stage circuit 203 is for example but not limited to asynchronous or asynchronous buck, boost, inverting, buck-boost,inverting-boost, and flyback power stage circuit as shown in FIGS.9A-9K. The control circuit 205 includes a differential amplifier circuit2051 and a pulse width modulation (PWM) signal generator 2053, whereinthe differential amplifier circuit 2051 has one input terminal whichreceives a current sense signal CS related to an output current Iout,and has another input terminal which receives a signal related to theinput voltage Vin. The differential amplifier circuit 2051 compares thecurrent sense signal CS and the signal related to the input voltage Vin,and amplifies their difference to generate an amplified signal which issent to the PWM signal generator 2053, whereby the PWM signal generator2053 generates the driving signal GATE. In this embodiment, the inputvoltage Vin is divided by resistors R1 and R2 connected in series, and acapacitor C1 is provided to stabilize the divided voltage inputted tothe input terminal of the differential amplifier circuit 2051. Thisarrangement is preferable but not absolutely necessary. For example, theinput voltage Vin may be inputted to the differential amplifier circuit2051 directly without being divided by the resistors R1 and R2, orwithout being stabilized by the capacitor C1, if the differentialamplifier circuit 2051 can sustain a high voltage or the fluctuation isnot a concern. By feedback control mechanism, the levels of the twoinput terminals of the differential amplifier circuit 2051 will be equalto each other (assuming that an internal bias of the differentialamplifier circuit 2051 can be ignored) when the circuit is at a balancedstate. Therefore, the current sense signal CS will follow the inputvoltage Vin, i.e., the phase of the output current Iout will follow thephase of the input voltage Vin, and thus the PFC function is achieved inthis embodiment.

In the power supply circuit 200 of the first embodiment, the amplitudeor the average value of the output current Iout will change if thespecification of the voltage Vac is different, such as a differentamplitude of 110V or 220V. To further improve this, the presentinvention provides the following embodiments wherein not only the phaseof the output current Iout is the same as the phase of the input voltageVin, but also an amplitude or average of the output current Iout is keptthe same even if the voltage Vac has a different specification, byproviding an automatic gain control circuit in the power supply circuit.

FIG. 2B shows a second embodiment of the present invention. As shown inFIG. 2B, a power supply circuit 200 includes an automatic gain controlcircuit 210 and a load driver circuit 220, wherein the load drivercircuit 220 includes for example but not limited to the power stagecircuit 203 and the control circuit 205 as shown in the firstembodiment. The automatic gain control circuit 210 converts the inputvoltage Vin to a regulation voltage Vrsin. In the process of convertingthe input voltage Vin to the regulation voltage Vrsin, the regulationvoltage Vrsin is in phase with the input voltage Vin, but a conversionratio between the input voltage Vin and the regulation voltage Vrsin isautomatically adjusted, such that the regulation voltage Vrsin has asubstantially fixed amplitude or average value even if the specificationof the input voltage Vin is different. The term “substantially fixed” isused because there may be factors affecting the amplitude or average ofthe regulation voltage Vrsin such as noises, etc., and the term“substantially fixed” intends to mean that the amplitude or average ofthe regulation voltage Vrsin does not significantly change beyond atolerable error.

Note that, in the process of converting the input voltage Vin to theregulation voltage Vrsin by the automatic gain control circuit 210, theautomatic gain control circuit 210 does not obtain any feedback signalfrom the load driver circuit 220; that is, the automatic gain controlcircuit 210 and the load driver circuit 220 are electrically connectedto form an open loop connection. This arrangement has an advantage that:a designer can design the load driver circuit 220 simply based on therequirements for driving the load, such that the settings of theparameters of the load driver circuit 220 are much simpler as comparedwith the case wherein the load driver circuit 220 is required to providea feedback signal to the automatic gain control circuit 210. Forexample, if the load is a light emitting diode (LED) circuit, the loaddriver circuit 220 may be an LED driver circuit which is deigned todrive the LED circuit, and it is only required to connect an inputterminal of the differential amplifier circuit 2051 to the regulationvoltage Vrsin, without changing any other parameter setting of the LEDdriver circuit.

FIG. 3 shows a third embodiment of the present invention. Thisembodiment shows a more specific embodiment of the automatic gaincontrol circuit. As shown in FIG. 3, a power supply circuit 300 includesan automatic gain control circuit 305 and a load driver circuit 303. Theautomatic gain control circuit 305 for example can be applied to thesecond embodiment as the automatic gain control circuit 210, and theload driver circuit 303 for example has the structure of the load drivercircuit 220 of the second embodiment. The automatic gain control circuit305 includes a voltage-to-current converter circuit (V-I Ckt.) 3051, acurrent-to-voltage converter circuit (I-V Ckt.) 3052, a low-pass filtercircuit (LPF Ckt.) 3053, and a differential amplifier circuit A1. Thecurrent-to-voltage converter circuit 3051 converts the input voltage Vinto a regulation current. The current-to-voltage converter circuit 3052is coupled to the voltage-to-current converter circuit 3051, forconverting the regulation current to the regulation voltage Vrsin. Inthe process of converting the input voltage Vin to the regulationcurrent, then to the regulation voltage Vrsin, the regulation voltageVrsin and the input voltage Vin are in phase, and a conversion ratiofrom the input voltage Vin to the regulation current, or a conversionratio from the regulation current to the regulation voltage Vrsin, isautomatically adjusted, such that the regulation voltage Vrsin has asubstantially fixed amplitude. The load driver circuit 303 for examplemay include the power stage circuit 203 and the control circuit 205 asshown in FIG. 2B, to convert the regulation voltage Vrsin to the outputvoltage Vout and to supply the output current Iout, so that the outputcurrent Tout has the same phase as the regulation voltage Vrsin, i.e.,the same phase as the input voltage Vin. The low-pass filter circuit3053 is coupled to the current-to-voltage converter circuit 3052, forobtaining a low frequency signal from the regulation voltage Vrsin,which can be regarded as obtaining an average of the regulation voltageVrsin. The differential amplifier circuit A1 is coupled to the low-passfilter circuit 3053, for generating a control signal according to thelow frequency signal and a reference voltage Vref. The control signalmay be fed back to the voltage-to-current converter circuit 3051 or thecurrent-to-voltage converter circuit 3052 to automatically adjust theconversion ratio from the input voltage Vin to the regulation current,or the conversion ratio from the regulation current to the regulationvoltage Vrsin. By feedback control mechanism, the levels of the twoinput terminals of the differential amplifier circuit A1 will be equalto each other (assuming that an internal bias of the differentialamplifier circuit A1 can be ignored) when the circuit is at a balancedstate. Therefore, the feedback control loop controls the average of theregulation voltage Vrsin to be substantially equal to the referencevoltage Vref; that is, the regulation voltage Vrsin has a substantiallyfixed average value regardless of the different amplitude of the inputvoltage Vin. Thus, if there is no other factor changing the amplitude ofthe regulation voltage Vrsin, the amplitude of the regulation voltageVrsin will have a substantially fixed amplitude; if there is any factorchanging the amplitude of the regulation voltage Vrsin (such as changingits peak or valley level), the average value of the regulation voltageVrsin still maintains unchanged even though its amplitude is changed.

FIG. 4 shows a fourth embodiment of the present invention. As shown inFIG. 4, a power supply circuit 400 includes an automatic gain controlcircuit 405 and the load driver circuit 303. The automatic gain controlcircuit 405 includes a voltage-to-current converter circuit 4051, acurrent-to-voltage converter circuit 4052, a low-pass filter circuit4053, and a differential amplifier circuit A1. The voltage-to-currentconverter circuit 4051 includes a differential amplifier circuit A2, atransistor SW2, and a variable resistor circuit 4054. The differentialamplifier circuit A2 is for generating a differential amplified signalaccording to the input voltage Vin and a regulation current feedbacksignal. The transistor SW2 is coupled to the differential amplifiercircuit A2, which operates to generate a regulation current Ia accordingto the differential amplified signal. The variable resistor circuit 4054is coupled to the transistor SW2, for generating the regulation currentfeedback signal according to the regulation current Ia. As shown in thefigure, the differential amplifier circuit A2 receives a divided voltagesignal of the input voltage Vin (a voltage drop across a resistor R4),and compares it with the regulation current feedback signal, to generatethe differential amplified signal which operates the transistor SW2.Therefore, the regulation current Ia is equal to the divided voltage(the voltage drop across the resistor R4) of the input voltage Vindivided by a resistance of the variable resistor circuit 4054. Thevariable resistor circuit 4054 includes for example but not limited to ametal oxide semiconductor (MOS) device as shown in the lower right ofthe figure, which has a resistance controllable by the control signal.In other words, when the control signal changes the resistance of theresistor circuit 4054, the regulation current Ia is correspondinglychanged, so that the conversion ratio from the input voltage Vin to theregulation current is automatically adjusted. Under the same spirit ofthe present invention, the conversion ratio from the input voltage Vinto the regulation current can be adjusted in various other ways. Forexample, in the circuit shown in FIG. 4, one or both of the resistors R3and R4 can be replaced by a variable resistor(s), and the control signalcan adjust the resistance of the variable resistor(s) to adjust theconversion ratio.

Referring to FIG. 4, in this embodiment, the current-to-voltageconverter circuit 4052 includes: a current mirror circuit 4055 and animpedance circuit 4056. The current mirror circuit 4055 is coupled tothe transistor SW2, for generating a duplicated regulation current Ia′by duplicating the regulation current Ia. The impedance circuit 4056 isfor example but not limited to a resistor as shown in the figure, whichis coupled to the current mirror circuit 4055, for converting theduplicated regulation current Ia′ to the regulation voltage Vrsin. Thelow-pass filter circuit 4053 may be an RC series circuit as shown in thefigure.

FIG. 5 shows a fifth embodiment of the present invention. Thisembodiment is different from the fourth embodiment in that, as shown inFIGS. 4 and 5, the variable resistor circuit 4054 and the impedancecircuit 4056 in the fourth embodiment are replaced by a resistor 4058and a variable resistor circuit 4059 in this embodiment, respectively.And in this embodiment, the control signal, which is generated by thedifferential amplifier circuit A1, controls a resistance of the variableresistor circuit 4059 to automatically adjust the conversion ratio fromthe regulation current to the regulation voltage Vrsin. The variableresistor circuit 4054 includes for example but not limited to a MOSdevice as shown in lower right of the figure, wherein its resistance iscontrollable by the control signal. In this embodiment, the regulationvoltage Vrsin also has a substantially fixed average and/or fixedamplitude under different input voltages Vin. Under the same spirit ofthe present invention, the conversion ratio from the regulation currentto the regulation voltage Vrsin can be adjusted in various other ways.For example, in the circuit of FIG. 5, the control signal may control aduplication ratio (from Ia to Ia′) of the current mirror circuit 4055(such as by changing a dimension of any transistor in the current mirrorcircuit 4055), and the regulation voltage Vrsin can be automaticallyadjusted thereby. For another example, the embodiments shown in FIGS. 4and 5 may be combined, such that the control signal may control both thevoltage-to-current converter circuit 4051 and the current-to-voltageconverter circuit 4052.

FIG. 6 shows a sixth embodiment of the present invention. Thisembodiment shows a more specific embodiment of the load driver circuit603. As shown in the figure, the load driver circuit 603 has adifferential amplifier circuit A3, a power stage circuit 6031, and acurrent sense circuit 6032. The differential amplifier circuit A3generates the differential amplified signal according to the regulationvoltage Vrsin and the current sense signal CS. The power stage circuit6031 operates at least one power switch according to the differentialamplified signal to convert the input voltage Vin or the regulationvoltage Vrsin to the output voltage Vout, which is supplied to a load10. The load 10 is for example but not limited to an LED circuit 10 asshown in the figure. Assuming that the output voltage Vout is notsupplied to any other circuit, the output current Iout is equal to anLED current ILED. If the output voltage Vout is supplied to another loadbesides the LED circuit 10, then the LED current ILED is still relatedto the output current Iout with known relationship. The current sensecircuit 6032 is provided in the loop of the LED current ILED (e.g., atthe upper or lower end of the LED circuit 10), and generates the currentsense signal according to the LED current ILED; the current sense signalis inputted to the differential amplifier circuit A3. By thisarrangement, the LED current ILED is in phase with the regulationvoltage Vrsin, according to the basic mechanism of feedback control.Because of the substantially fixed average value and/or fixed amplitudeof the regulation voltage Vrsin, the LED current ILED also has asubstantially fixed average value and/or fixed amplitude. Therefore, theLED circuit 10 can maintain the same brightness even if the inputvoltages Vin has a different specification.

FIG. 7 shows a seventh embodiment of the present invention. Thisembodiment shows a more specific embodiment of the load driver circuit703. As shown in the figure, the load driver circuit 703 includes thedifferential amplifier circuit A3, a power stage circuit 6031, and acurrent sense circuit 7032. This embodiment is different from the sixthembodiment in that, in this embodiment, the current sense circuit 7032for example may sense the output current Iout instead of the LED currentILED.

FIG. 8 shows an eighth embodiment of the present invention. As shown inFIG. 8, an automatic gain control circuit 9045 includes avoltage-to-current converter circuit 9051, a current-to-voltageconverter circuit 9052, the low-pass filter circuit 4053, and thedifferential amplifier circuit A1. This embodiment is different from thefourth embodiment (shown in FIG. 4) in that, in this embodiment, thevoltage-to-current converter circuit 9051 further includes an upperlimit gain determination circuit 9053 and a lower limit gaindetermination circuit 9054 besides the differential amplifier circuitA2, the transistor SW2, and the feedback circuit 4054. The upper limitgain determination circuit 9053 is connected with the variable resistorcircuit 4054 in series, for determining an upper limit of the gain ofthe automatic gain control circuit 905. The upper limit gaindetermination circuit 9053 is for example but not limited to a resistoras shown in the figure. The lower gain limit determination circuit 9054is connected with the variable resistor circuit 4054 in parallel, fordetermining a lower limit of the gain the automatic gain control circuit905. More specifically, assuming that the variable resistor circuit 4054is a MOS device as shown in the figure, when the MOS device is turnedOFF, the resistance of the MOS device is infinite, and in this case theconversion ratio from the input voltage Vin to the regulation current isdetermined by the resistance of the lower gain limit determinationcircuit 9054. On the other hand, when the MOS device is completelyturned ON, the resistance of the MOS device is zero, and in this casethe conversion ratio from the input voltage Vin to the regulationcurrent is determined by the resistance of the parallel circuit formedby the upper gain limit determination circuit 9053 and the lower gainlimit determination circuit 9054.

In addition, referring to FIG. 9, this embodiment is further differentfrom the fourth embodiment (as shown in FIG. 4) in that, in thisembodiment, the current mirror circuit 9052 further includes a constantcurrent source 9055, wherein the constant current source 9055 is fordetermining a minimum level (valley) of the regulation voltage Vrsin.More specifically, the regulation current Ia, the duplicated regulationcurrent Ia′, and a current Ib generated by the constant current source9055 have a relationship of: Ia′=Ia+Ib. In other words, when theregulation current Ia is zero, the current Ib determines a minimum levelof the duplicated regulation current Ia′, and therefore the minimumlevel (valley) of the regulation voltage Vrsin is determined. When thevalley of the regulation voltage Vrsin is determined, because itsaverage value is substantially fixed, its amplitude will changecorrespondingly; that is, when its valley is increased, its peak isdecreased, and when its valley is decreased, its peak is increased.

Certainly, the method of determining the upper gain limit and the lowergain limit may be applied to the fifth embodiment (as shown in FIG. 5)as well. For example, the upper gain limit determination circuit 9053may be added in series connection to the variable resistor circuit 4059,and the lower gain limit determination circuit 9054 may be added inparallel connection to the variable resistor circuit 4059.

The present invention has been described in considerable detail withreference to certain preferred embodiments thereof. It should beunderstood that the description is for illustrative purpose, not forlimiting the scope of the present invention. Those skilled in this artcan readily conceive variations and modifications within the spirit ofthe present invention. For example, the differential amplifier circuitA1 may be changed to a comparator circuit, which generates the controlsignal in digital form to control the variable resistor with digitallogic. Therefore in the context of the specification and the claims, itis intended for the term “comparison circuit” to be a genius termincluding the species of the differential amplifier circuit and thespecies of the comparator circuit. For another example, a device whichdoes not substantially influence the primary function of a signal can beinserted between any two devices in the shown embodiments, such as aswitch or the like, so the term “couple” should be a genius termincluding the species of direct connection and the species of indirectconnection. For another example, the positive and negative inputterminals of the comparison circuits are interchangeable, withcorresponding amendment of the circuits processing these signals. Inview of the foregoing, the spirit of the present invention should coverall such and other modifications and variations, which should beinterpreted to fall within the scope of the following claims and theirequivalents.

1-25. (canceled)
 26. A control method of an automatic gain controlcircuit for use in a power supply circuit, wherein the power supplycircuit is for converting an input voltage to an output voltage andsupplying an output current, wherein the output current is in phase witha regulation voltage, the control method comprising: converting theinput voltage to a regulation current; converting the regulation currentto a regulation voltage; filtering the regulation voltage to obtain afiltered signal; and generating a control signal according to thefiltered signal; wherein the control signal is for adjusting aconversion ratio from the input voltage to the regulation current, orfrom the regulation current to the regulation voltage.
 27. The controlmethod of claim 26, further comprising: determining an upper limitconversion ratio and/or a lower limit conversion ratio from the inputvoltage to the regulation current to determine an upper limit of a gainand/or a lower limit of a gain of the automatic gain control circuit.28. The control method of claim 26, further comprising: determining anupper limit conversion ratio or a lower limit conversion ratio from theregulation current to the regulation voltage to determine an upper limitof a gain or a lower limit of a gain of the automatic gain controlcircuit.
 29. The control method of claim 26 further comprising:determining a minimum level of the regulation voltage.
 30. A powersupply circuit, for generating an output current according to an inputvoltage, wherein the output current has a substantially same averagevalue under different input voltages of different specifications, thepower supply circuit comprising: an automatic gain control circuit, forgenerating a regulation signal which is in phase with the input voltage,wherein the regulation signal has a substantially same amplitude oraverage value under different input voltages of differentspecifications; and a load driver circuit, controlled by the regulationsignal with an open loop connection, for generating the output current.31. The power supply circuit of claim 30, wherein the load drivercircuit includes a control circuit and a power stage circuit, whereinthe control circuit receives the regulation signal with an open loopconnection and generates a driving signal for controlling the powerstage circuit to generate the output current according to the regulationsignal.
 32. The power supply circuit of claim 30, wherein the automaticgain control circuit includes: a voltage-to-current converter circuitfor converting an input voltage to a regulation current; acurrent-to-voltage converter circuit, which is coupled to thevoltage-to-current converter circuit, for converting the regulationcurrent to a regulation voltage; a low-pass filter circuit, which iscoupled to the current-to-voltage converter circuit, for obtaining a lowfrequency signal from the regulation voltage; and a comparison circuit,which is coupled to the low-pass filter circuit, for generating acontrol signal according to the low frequency signal and a referencevoltage to control a parameter in the voltage-to-current convertercircuit, such that the regulation voltage maintains at a substantiallysame amplitude or average value under different input voltages ofdifferent specifications, wherein the regulation signal is theregulation voltage or a signal derived from the regulation voltage. 33.The power supply circuit of claim 30, wherein the automatic gain controlcircuit includes: a voltage-to-current converter circuit for convertingan input voltage to a regulation current; a current-to-voltage convertercircuit, which is coupled to the voltage-to-current converter circuit,for converting the regulation current to a regulation voltage; alow-pass filter circuit, which is coupled to the current-to-voltageconverter circuit, for obtaining a low frequency signal from theregulation voltage; and a comparison circuit, which is coupled to thelow-pass filter circuit, for generating a control signal according tothe low frequency signal and a reference voltage to control a parameterin the current-to-voltage converter circuit, such that the regulationvoltage maintains at a substantially same amplitude or average valueunder different input voltages of different specifications, wherein theregulation signal is the regulation voltage or a signal derived from theregulation voltage.
 34. The power supply circuit of claim 32, whereinthe comparison circuit is a differential amplifier circuit.
 35. Thepower supply circuit of claim 33, wherein the comparison circuit is adifferential amplifier circuit.